Aug. 29, 2017
Did Ice Age Cause Mastodon Extinction? New Research Suggest Several Causes
About 10,000 years ago, in the last moments of the Pleistocene epoch, an extinction of large mammals, or megafauna, occurred. These included the woolly mammoths, mastodons, saber-tooth tigers, giant ground sloths, and enormous woolly bears, all North American inhabitants of the last major ice age.
"There's been a lot of discussion on whether climate change or human over-hunting caused the mass extinction," said Meaghan Emery-Wetherell, Central Washington University Associate Director of Institutional Effectiveness and lecturer in Geological Sciences. "However, it has been hard to determine since both people and climate change (the end of the last ice age) appear at about the same time."
Emery-Wetherell, and colleagues Brianna McHorse, of Harvard University, and Edward Davis, of the University of Oregon Museum of Natural and Cultural History, recently published "Spatially Explicit Analysis Sheds New Light on the Pleistocene Megafaunal Extinction in North America," in Paleobiology, in which they present an exciting new methodology for finding clues to why these massive creatures died out.
Looking strictly at North America, the researchers employed the technique of using heat maps of the region over thousands of years, and overlaying the maps with data recording the last occurrences of the megafauna—and the first signs of humans. The data included maps of human and animal carbon-dated fossil discoveries during the same time period.
"No one has ever used this combination of geography and temporal information before," Emery-Wetherell noted. "We looked at overlapping occurrences of human and animal species, from the last appearance of megafauna and the first appearance of humans."
Emery-Wetherell and her colleagues found different scenarios in different regions. In the upper latitudes, in what is currently Alaska, megafauna went extinct before humans showed up, she explained, during an intense cold period that came before end of the ice age. It appears that the extinction there was due to colder temperatures and loss of habitat.
The pattern of extinction in areas further south, however, followed the climate heating at the end of the ice age itself. "In the main body of the United States, you see megafaunal extinctions tracking changes in the ecosystem," Emery-Wetherell noted. "The cool mixed forest that megafauna thrived in started to shrink. The vast, partially wooded grasslands, from the Great Lakes region to what is now Texas, were gradually opening up and changing.
"As their biome became more destabilized, and megafauna populations began shrinking due to decreasing food supplies, human predation had a more devastating effect."
The study also identified two regions where humans and megafauna may have coexisted for up to 7,000 years before going extinct, in the Great Lakes, and in southern Texas and northern Mexico. Such long periods of coexistence put to rest the theory that the megafaunal extinction was entirely due to human over-hunting. Humans had an effect on the mass extinction of megafauna, yet the precipitating, and far more lethal, event was the critical loss of habitat due to climate change. Even in areas where humans and megafauna co-existed, once the habitat the megafauna relied upon was gone, so too went the megafauna.
"And while climate change started this extinction event, the growing presence of humans intensified its effect," Emery-Wetherell said. "Studies of other regions show varying rates of megafauna decline that coincide with human presence. A combination of factors, a perfect storm of climate change and the introduction of a new predator—humans— destroyed the enormous creatures that used to roam the continent."
Paleobiology will provide three weeks of free access to the article at http://www.cambridge.org/pab/wetherell.
Illustration by Mauricio Antón
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